Optical transmitter

ABSTRACT

In order to provide an optical transmitter applicable for a transmission speed range higher than 1 Gb/s without transmission quality degradation, the embodiment of the invention comprises a flipflop having a set terminal supplied with a transmission signal and a variable delay circuit for delaying an inverse output of the flipflop to be supplied to a reset terminal of the flipflop, for equalizing pulse width of the laser light with that of the transmission signal.

This application is a division of application Ser. No. 08/612,417 filedMar. 07, 1996 now Pat. No. 5,880,869.

BACKGROUND OF THE INVENTION

This invention relates to an optical transmitter, and more particularlyto an optical transmitter with a laser diode of low threshold currentdensity.

Recently, optical transmission technology has been developed remarkablyand ultra high speed optical transmission technologies by way of asingle mode optical fiber, an optical transmission device for a longwave laser light, are investigated for realizing a large capacity longdistance transmission.

Especially for providing a wide area communication network used fordelivering multimedia services such as video picture, audio sound orinformation data, a high-speed, stable and practical optical transmitteris expected.

In the wide area communication network, not only for the trunk lines,where time division multiplex signals of several giga-bits per secondare transmitted requiring a high-speed and wide band transmitter, butalso for subscriber lines, these optical transmission technologies areattracting considerable attention as a feasible strategy for providing ahigh quality subscriber line interface for multimedia services.

Basic functions needed of the optical transmitter/receiver are so called3R, that is, resharping (or equivalent amplification), retiming andregeneration. In the regeneration among them, the optical transmitterplays an important role to send out final transmission signals.

Therefore, cost reduction of the optical transmitter is a hot theme fordevelopement of subscriber communication systems.

As for the cost reduction of optical transmitters, a regulation freesetting of bias current of a laser diode, a simplification of a drivingcircuit of the laser diode or a reduction of power consumption of thecircuit has been investigated, and recentry as a result of developementof a low threshold laser diode, a zero-bias modulation is realized,which is reported in "Zero-bias Modulation of extremely Low Threshold1.3 μm DFB-PPIH laser diode", Ohkura et al, Optical CommunicationSociety 88-15, pp. 37 to 41.

FIG. 5 illustrates a circuit configuration of a conventional opticaltransmitter using a comparatively low threshold current density laserdiode as its optical source. Input signals are amplified by a laserdiode driver (hereafter abbreviated to a LD driver) 51 to a signal levelsufficient for activating a laser diode (hereafter abbreviated to a LD)52, which sends out optical outputs 54 according to driving pulses 53supplied from the LD driver 51. Wave forms of a driving pulse 53 and acorresponding optical output 54 are shown in FIG. 6.

In general, when a LD is used without bias, it causes a radiation delay,which depends on threshold value of the LD and becomes longer with thehigher threshold value. Therefore, a radiation delay longer than asignal period may cause information defects. This is an importantproblem of the LD.

The radiation delay td of a LD, used with bias lower than its thresholdvalue, is given by a following equation;

    td=Ts×ln{Jp/(Jp-Jth+Jb)}                             (1)

where, Ts, Jth, Jb and Jp denote carrier lifetime, threshold currentvalue, bias current value and driving current value respectively, thedriving current value defined as a current value difference between apeak current and the bias current.

From equation (1), the radiation delay td of about 250 ps is given in aLD with a threshold current value Jth of 3.5 mA activated by a drivingcurrent Jp of 40 mA without bias current. So, in a conventional opticaltransmitter as shown in FIG. 5 with a comparatively low threshold LD 52,the optical output 54 is delayed from the driving pulse 53 as shown inFIG. 6.

The radiation delay td of a LD represented by equation (1) is oncondition that the LD is activated with a driving pulse after a certaininterval. The radiation delay td becomes shorter when the LD isactivated with a driving pulse following other preceding pulses thanthat after the interval, since carrier density in active layers of theLD 52 widely fluctuates depending on pulse patterns of the driving pulse53.

Consequently, a radiation delay td1 for a first pulse #1 of the opticaloutput 54 in FIG. 6, which equals td obtained from the equation (1), islonger than the next radiation delay td2 for a following pulse #2.

This fluctuation of the radiation delay results in jitters of risingedges of the optical output 54, and when transmission speed becomeshigh, more than 1 Gb/s for example, influences of the radiation delay tdand the jitters become important compared to pulse widths, degrading thetransmission characteristic and limiting the errorless transmissionspeed.

In prior art reducing the radiation delay td, a LD bias circuit 74 isprovided as illustrated in FIG. 7 for supplying a bias current to the LD52 as shown in FIG. 8. However, the bias current of the prior artcancels the merit of low dissipation of the comparatively low thresholdLD 52 and complicates the circuit configration of the opticaltransmitter.

SUMMARY OF THE INVENTION

Therefore, a primary object of the present invention is to materializean optical transmitter which can transmit optical signals according totransmission data of various speeds higher than 1 Gb/s, without qualitydegradation with a simple circuit of low dissipation.

Another object of the invention is to materialize an optical transmitterwhich can transmit optical signals according to transmission data ofvarious speeds, with few pattern jitters.

In order to achieve the objects, an optical transmitter of an embodimentof the present invention with a laser diode having a low threshold oldcharacteristic for emitting a laser light according to a pulse currentsupplied corresponding to a transmission signal comprises equivalencemeans for revising a pulse width of the transmission signal forequalizing a pulse width of the laser light to a pulse width of theoriginal transmission signal.

Therefore, the decrease of pulse widths of the optical output because ofthe radiation delay is eliminated, resulting in a stable transmission.

An optical transmitter of another embodiment of the present inventionfurther comprises variable amplitude laser diode driver for amplifyingthe transmission signal revised by the equivalence means for supplyingto the laser diode with an amplitude determined according to a bit rateof the transmission signal.

Therefore, the errorless transmission speed range higher than 1 Gb/s canbe enlarged of the optical transmitter of the embodiment with a lowthreshold LD and a simple circuit of low dissipation, by decreasing thepattern jitters caused by the radiation delay depending on pulsepatterns.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing, further objects, features, and advantages of thisinvention will become apparent from a consideration of the followingdescription, the appended claims, and the accompanying drawings in whichthe same numerals indicate the same or the corresponding parts.

FIG. 1 is a block diagram of an optical transmitter of a firstembodiment of the invention.

FIG. 2 is a timing chart illustrating operation of the opticaltransmitter of FIG. 1.

FIG. 3 shows a second embodiment of the invention.

FIG. 4 shows a third embodiment of the invention.

FIG. 5 illustrates a circuit configuration of a conventional opticaltransmitter.

FIG. 6 shows wave forms of a driving pulse 53 and a correspondingoptical output 54 of FIG. 5.

FIG. 7 illustrates a block diagram of a prior art with a LD biascircuit.

FIG. 8 shows a characteristic of a LD activated with a driving pulsebiased.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In connection with the drawings, embodiments of the present inventionwill be described in the following paragraphs.

The embodiments are applied in an optical transmitter wherein a LD ofcomparatively low threshold current density is activated by a drivingcurrent composed of pulses corresponding to a transmission signal.

FIG. 1 is a block diagram of an optical transmitter of a firstembodiment of the invention, and FIG. 2 is a timing chart illustratingoperation of the optical transmitter of FIG. 1.

Referring to FIG. 1, the optical transmitter comprises a flipflop 13supplied with a transmission signal, a delay circuit 14 for delaying aninverse output of the flipflop 13 to be supplied to the flipflop 13 as areset signal, a LD driver 11 for amplifying output of the flipflop 13 toa desired level for activating a comparatively low threshold LD 12.

Delay time of the delay circuit 14 is variable and controlled so as tomake pulse widths of optical output of the LD 12 equal to pulse widthsof the transmission signal. Therefore, optical output having same pulsewidths with those of an input transmission signal is obtained in a rangehigher than 1 Gb/s without degrading the transmission quality.

The transmission signal, which is illustrated by a first pattern PS inFIG. 2, is input to a set terminal S of the flipflop 13. Output signalfrom output terminal Q is amplified by the LD driver 11 to a sufficientlevel for activating the LD 12.

Now, operation of the optical transmitter of FIG. 1 is described inconnection with FIG. 2.

Outputs of the Q and the Q terminals of the flipflop 13 become at logicHIGH and logic LOW respectively at a rising edge of the transmissionsignal, as the first pattern PS at a timing #1 of FIG. 2, and theoptical output of the LD 12 becomes at logic HIGH after the radiationdelay td represented by the equation (1). As described above theradiation delay td is about 250 ps when the threshold current Jth andthe driving current Jp of the LD 12 is 3.5 mA and 40 mA respectively.Output of the Q terminal of the flipflop 13. is input to the resetterminal R of the flipflop 13 after being delayed for a certain time bythe delay circuit 14 as a fourth pattern PR of a timing #2 illustratedin FIG. 2. So, outputs of the Q and the Q terminals of the flipflop 13turn to logic LOW and logic HIGH respectively and the optical outputbecomes at logic LOW, at timings #3 and #4 of FIG. 2.

Therefore, T being a pulse width, by setting the delay time of the delaycircuit 14 to 100 ps+T, optical output having the same pulse widths Twith those of the transmission signal supplied to the opticaltransmitter is obtained, since transfer delay Tpd through the flipflop13 is about 150 ps when the a high-speed flipflop is applied to theflipflop 13.

Thus, the decrease of pulse widths of the optical output because of theradiation delay is eliminated, resulting a stable transmissioncharacter.

FIG. 3 shows a second embodiment of the invention, where the LD driver11 of FIG. 1 is replaced with a variable amplitude LD driver 31.

In the second embodiment, the pattern jitters of the optical output fromthe LD 12 are reduced by applying the variable amplitude LD driver 31 inaddition to the elimination of the pulse width decrease, providing astill excellent transmission quality in the range higher than 1 Gb/swithout additional circuit for supplying bias current to the LD 12.

It can be seen from the equation (1) that the radiation delay of theoptical output of the LD 12 depends also on the driving current Jpsupplied to the LD 12 from the variable amplitude LD driver 31. Theradiation delay td decreases by increasing the driving current Jp. Itmeans that by setting an appropriate amplitude according to the bit rateof the transmission signal, fluctuation of carrier density in the activelayers of the LD 12 can be prevented, resulting in a decrease of thepattern jitters caused by the radiation delay depending on pulsepatterns with a simple circuit of low dissipation.

Thus, the errorless transmission speed range higher than 1 Gb/s can beenlarged of the optical transmitter with a low threshold LD.

FIG. 4 shows a third embodiment of the invention, wherein a delay timecontroller 45 and a driving amplitude controller 46 are provided inaddition to the optical transmitter of FIG. 3 for controlling the delaycircuit 14 and the variable amplitude LD driver 31 according to controlsignals C1 and C2 respectively.

So, by supplying appropriate signals C1 and C2 in accordance withtransmission speeds, an optical transmitter applicable for varioustransmission speeds is realized with a high quarity transmissioncharacteristic without pulse width decrease and pattern jitters, even ina range higher than 1 Gb/s.

What is claimed is:
 1. An optical transmitter comprising:a laser diodehaving a low threshold characteristic for emitting a laser lightaccording to a pulse current corresponding to a transmission signal;equivalence means for egualizing a pulse width of said laser light witha pulse width of said transmission signal, wherein said equivalencemeans revises said pulse width of said transmission signal; and avariable amplitude laser diode driver for amplifying said transmissionsignal revised by said equivalence means to supply to the laser diode anamplitude determined according to a bit rate of said transmissionsignal.
 2. An optical transmitter comprising:laser diode having a lowthreshold characteristic for emitting a laser light according to a Pulsecurrent corresponding to a transmission signal; equivalence means forequalizing a pulse width of said laser light with a pulse width of saidtransmission signal, wherein said equivalence means revises said pulsewidth of said transmission signal; and a variable amplitude laser diodedriver for amplifying said transmission signal revised by saidequivalence means to supply to the laser diode an amplitude according toan amplitude control signal.
 3. An optical transmitter as recited inclaim 1, wherein said equivalence means comprises:a flipflop having aset terminal supplied with said transmission signal; and a variabledelay circuit for delaying an inverse output of said flipflop to besupplied to a reset terminal of said flipflop.
 4. An optical transmitteras recited in claim 2, wherein said equivalence means comprises:aflipflop having a set terminal supplied with said transmission signal;and a variable delay circuit for delaying an inverse output of saidflipflop to be supplied to a reset terminal of said flipflop.